Recent experiments in ionic crystals reveal that (in the absence of any external electric field) a time-dependent electric polarization arises upon changing the rate of the uniaxial stress or by the indenter penetration into the crystal surface. The latter experiments lead to activation volumes, which are order(s) of magnitude smaller than those measured by Lazarus and co-workers [Phys. Rev. B 5, 4935 (1972); Phys. Rev. B 8, 1726 (1973)]. We show that this difference is not inconsistent with thermodynamical concepts of point defects. Furthermore, an explanation of the nondetection of the cofracture electric signals at large distances, when the crystal is surrounded by a weakly conducting medium, is suggested.

• Received 30 October 1997

DOI:https://doi.org/10.1103/PhysRevB.59.24